1. Field of the Invention
The present invention relates to an apparatus and method for embedding embedment information as digital watermarks into an image signal, an apparatus and method for detecting the digital watermarks from the image signal, and a medium having a program recorded therein for practicing each of the above methods.
2. Description of the Related Art
In recent years, digital content such as digitized audio and digitized video data has been on the increase. The digital content makes it easy to provide a faithful reproduction of original content. Therefore, it is an important issue to protect the copyright of the digital content. Illegally reproduced or distributed content is very difficult to differentiate from corresponding original content. This fact involves difficulties in demonstrating an evidence to assert the copyright of the content. As a result, efforts have been made to protect the copyright of the digital content.
A “digital watermark” is used as one of the efforts. The digital watermark is an art of embedding data into the video data without allowing human beings to perceive degradation in image quality, and further of detecting the embedded data from the image data having the embedded data embedded therein.
A prior art digital watermark-embedding apparatus of the type discussed above has been disclosed in, e.g., published Japanese Patent Application Laid-Open No. 2000-175161 (patent reference No. 1).
This prior art is now described with reference to FIG. 18. FIG. 18 is a block diagram illustrating the prior art digital watermark-embedding apparatus.
In FIG. 18, a block-dividing unit 1401 divides a moving image frame (image data) into blocks, while a block-extracting unit 1402 extracts one of the blocks, which is designated by a template 1409.
A DCT unit 1403 practices the DCT processing of the extracted block. A DCT coefficient-extracting unit 1404 extracts a DCT coefficient designated by the template 1409. A watermark data-embedding unit 1405 increases an absolute value of a DCT coefficient value, and then embeds watermark data into the DCT coefficient value.
An inverse DCT unit 1406 performs the inverse DCT of the DCT coefficient value having the watermark embedded therein, and then feeds a block image into a block-combining unit 1407. The block-combining unit 1407 combines the block images together, thereby generating one-frame image data. The block-combining unit 1407 feeds the one-frame image data into an MPEG-encoding unit 1408.
The MPEG-encoding unit 1408 encodes the image data, and then outputs compressed image data. For example, an image database server delivers the compressed image data to a client.
However, the prior art requires the above-mentioned processes of DCT, inverse DCT, and MPEG-encoding in order to distribute the compressed image data having the digital watermarks embedded therein.
The embedded watermark data is detected in a manner as discussed below.
An absolute value of a DCT coefficient value of a block having watermark data embedded therein is compared with an absolute average value of a corresponding DCT coefficient value of a neighboring block. When the absolute value of the former DCT coefficient value differs from the absolute average value of the latter DCT coefficient value by an amount equal or greater than a scheduled threshold value, then it is detected that the watermark data have been embedded.
As a result, the embedded watermark data are difficult to detect when a block to be detected and its neighboring blocks have greatly different image complexities. For example, a flat image results in a DCT coefficient having a reduced amount of an alternating current component value. An image abundant with edges causes a DCT coefficient having an increased amount of alternating current component. According to the process as taught by patent reference No. 1, these different images intermingled in both of the block to be detected and the neighboring blocks bring about a problem of poor detective precision.
According to the process of patent reference No. 1, a template is used to determine a block where digital watermarks are embedded. This step may degrade image quality, depending on the complexity of a corresponding image of the determined block. For example, when the block determined by the template is a flat image, then digital watermark embedment results in deteriorated image quality.
Such a digital watermark-embedding method is taught in published Japanese Patent Application Laid-Open No. 11-75166 (patent reference No. 2). Patent reference No. 2 discloses a method for superposing a micro level of additional information on a several-pixel basis of a video signal.
According to patent reference No. 2, a superposing level of additional information within on a several-pixel basis is varied to permit a superposing level pattern within on the several-pixel basis to coincide with a predetermined invariable pattern, thereby superposing the additional information onto a video signal.
According to patent reference No. 2, each pixel on a pixel domain is minutely varied to embed digital watermarks, with the result of an increased amount of processing. For example, in order to embed the digital watermarks into an originally MPEG-encoded video, extension must be at first made to the pixel domain before re-encoding is carried out after the digital watermark embedment. This means that patent reference No. 2 is improper for real-time processing.